This invention relates to fiber optic scanning devices, such as fiber optic image acquisition devices and fiber optic image display devices, and more particularly to a fiber optic scanning device which achieves a high image resolution and a wide field of view using a flexible fiber of very small diameter.
Fiber optic image acquisition devices include endoscopes, boroscopes and bar code readers. An endoscope is an imaging instrument for viewing the interior of a body canal or hollow organ. Entry typically is through a body opening. A boroscope is an imaging instrument for viewing an internal area of the body. Entry typically is invasive through a `bored` opening (e.g., a surgical opening).
There are rigid endoscopes and flexible endoscopes. Rigid endoscopes do not have a pixelated image plane. Flexible endoscopes are smaller and conventionally have a pixelated image plane. Flexible endoscopes, however, are unable to achieve the resolution and field of view of rigid endoscopes. But the rigid endoscopes are unable to be used in many applications where small size and flexible fibers and shafts are required.
The goal of any endoscope is high image quality in a small package, allowing minimal tissue trauma. In the growing field of minimally invasive surgical techniques, there is great demand for smaller endoscopes that match current image quality. In particular, the demand for minimally invasive medical procedures has increased the demand for ultrathin optical endoscopes. However, commercial flexible endoscopes have a fundamental tradeoff of size versus image quality. The smaller the endoscope diameter the lower the image resolution and/or field-of-view (FOV), such that image quality deteriorates. Many endoscopic techniques are not possible or become risky when very small endoscopes are used because the doctor has insufficient visual information, i.e. small size and poor quality of images. Accordingly, there is a need for very small, flexible endoscopes with high resolution and FOV. This fundamental tradeoff of a flexible image generator that has both a very small diameter and has the high image quality is a major limitation in applications outside the human body, such as remote sensing.
Conventional flexible endoscopes and boroscopes include a large spatial array of pixel detectors forming a CCD camera. Typically a bundle of optical fibers capture an image and transmit the image to the CCD camera. To achieve a high resolution, wide field image, such CCD cameras often include a pixel detector array of approximately 1000 by 1000 detectors. For color fidelity it is common to include three such arrays, and where stereoscopic viewing is desired, this doubles to six arrays. A fiber is present for each pixel detector. Each fiber has a diameter greater than or equal to 4 microns. Thus, acquisition requires a space of greater than or equal to 4 microns per pixel. If a standard sVGA image is desired (800.times.600 pixels), then a minimum diameter of just the image conduit is greater than 3 mm. A 1000 by 1000 pixel detector array has a diameter of at least 4 mm. For a VGA standard, resolution and/or field of view is sacrificed by having fewer pixel elements in order to attain less than 3 mm overall diameter scopes. Reducing the diameter of the endoscope reduces the possible number of pixels, and accordingly, the resolution and field of view. Limits on diameter also limit the opportunity to access color images and stereoscopic images.
In the field of small (e.g., less than 3 mm dia.), flexible endoscopes, the scopes need to use the smallest pixel size, while still reducing the number of pixels, typically to (100.times.100). Note, these small flexible endoscopes are found by surgeons to be too fragile, so as not to be widely used. Instead doctors prefer small, but rigid-shafted (straight) endoscopes, greatly limiting their maneuverability and applicability.
In the field of large (e.g., greater than or equal to 4 mm dia.), flexible endoscopes, the scopes have a flexible shaft which is greater than or equal to 4 mm in diameter and typically include either a bundle of optical fibers or a small camera at the distal end to capture the image. However, there is still a tradeoff between the desired 50-70.degree. FOV and image resolution at the full potential of human visual acuity until the scope diameter reaches&gt;10 mm.
U.S. Pat. No. 5,103,497 issued Apr. 7, 1992 of John W. Hicks discloses a flying spot endoscope in which interspacing among fiber optics is decreased to reduce the overall diameter of the optical bundle. Rather than arrange a bundle of fibers in a coherent manner, in his preferred embodiment Hicks uses a multi-fiber whose adjacent cores are phase mismatched. The multi-fiber is scanned along a raster pattern, a spiral pattern, an oscillating pattern or a rotary pattern using an electromagnetic driver. The illumination fibers, the viewing fibers or both the illuminating fibers and the viewing fibers are scanned. In a simplest embodiment, Hicks discloses scanning of a single fiber (e.g., either the illuminating or the viewing fiber).
Hicks uses a small bundle or a single fiber to scan an image plane by scanning the fiber bundle along the image plane. Note that the image plane is not decreased in size. The smaller bundle scans the entire image plane. To do so, the bundle moves over the same area that in prior art was occupied by the larger array of collecting fiber optics. As a result, the area that Hicks device occupies during operation is the same as in prior devices. Further, the core size of the fibers in Hicks' smaller bundle limits resolution in the same manner that the core size of fibers in the prior larger arrays limited resolution.
One of the challenges in the endoscope art is to reduce the size of the scanning device. As discussed above, the minimal size has been a function of the fiber diameter and the combination of desired resolution and desired field of view. The greater the desired resolution or field of view, the larger the required diameter. The greater the desired resolution for a given field of view, the larger number of fibers required. This restriction has been due to the technique of sampling a small portion of an image plane using a fiber optic camera element. Conventionally, one collecting fiber is used for capturing each pixel of the image plane, although in Hicks one or more fibers scan multiple pixels.
When generating an image plane, an object is illuminated by illuminating fibers. Some of the illuminating light impinges on the object directly. Other illuminating light is scattered either before or after impinging on the object. Light reflected from the image plane is collected. Typically, the desired, non-scattered light reflected from an illuminated portion of an object is differentiated from the scattered light by using a confocal system. Specifically a lens focuses the light returning to the viewing fiber. Only the light which is not scattered travels along a direct path from the object portion to the lens and the viewing fiber. The lens has its focal length set to focus the non-scattered light onto the tip of the viewing fiber. The scattered light focuses either before or after the viewing fiber tip. Thus, the desired light is captured and distinguished from the undesired light. One shortcoming of this approach is that most of the illuminated light is wasted, or is captured by surrounding pixel elements as noise, with only a small portion returning as the non-scattered light used to define a given pixel.